Gas turbine exhaust ducts must be connected to a ducting system through an expansion joint which is designed to provide stress relief caused by sudden thermal changes. These expansion joints also act as vibration isolators and compensate for minor misalignment of interconnecting ducts. They are constructed from a variety of metal parts and non-metallic materials including synthetic elastomers, fabrics, insulation materials and plastics, depending on their designs. Such exhaust duct expansion joints also have many applications such as for use in smelters, incineration plants, power generation plants, gas turbine plants and many more.
Because these joints are subjected to very rapid increase in internal exhaust gas temperatures, they are subjected to large temperature differentials within the joint structures, and these thermal gradients can cause stress levels in excess of yield. When a turbine is cycled frequently, such as during commissioning or as in peaking power units, the net result is deformation, crack propagation and gas leakage in some of the joint component parts.
A typical expansion joint uses a flexible composite membrane made from insulating and elastomeric materials to give the joint flexibility and sealing and it is attached to a steel structural support frame. There is generally a temperature limit for the materials that is considerably below the gas turbine exhaust temperature. Hence the material is normally attached to the duct via a radial web about 5" to 6" (12 cm to 15 cm) deep. This web allows the temperature to reduce radially outwards so that the temperature at the material attachment point is about 400.degree. to 500.degree. F. (220.degree. to 280.degree. C.) below the gas temperature. The joint also acts as a transition piece between parts of the exhaust duct that is insulated on the inside to where the duct is insulated on the outside. The main area of high stress is the down stream structure that is insulated on the outside only. At this point the web of the fabric support flange restricts the radial growth of the duct. The stresses in the other half of the joint where non-continuous insulated floating boxes are used to protect the duct from high temperatures, are much less.
In my U.S. Pat. No. 5,378,026 issued on Jan. 3, 1995, I discuss the construction of a circular expansion joint. A circular joint behaves very differently than a rectangular duct joint. Unlike a circular joint with the same radial temperature gradient, each side of the rectangular joint acts as a beam with a temperature gradient across it. Each side tries to bow inwards due to the thermal gradient. This bowing movement is restricted by the restraint at each corner. If the corners fully restrain the sides from bending, the maximum bending stress would be ##EQU1## where .sigma..sub.b is the bending stress, E is the modulus of elasticity, .alpha. is the expansion coefficient, and T is the temperature difference.
This stress is less than the same stress for a circular joint, but it only applies away from the corners. At the corners the stress level in the web increases significantly. This is because at the corner, the direct (hoop) stress that is in the duct and outer flange cannot be transmitted around the corner, and this force is transferred to the web at that point. The bending moment that was carried by the whole cross section is now carried by the web only, and the duct and flange direct stress drop to a low value. The stress increase can therefor be approximated by the ratio of the effective moment of inertia of the full joint cross section to that of the web only. For a typical section, this ratio would be about 4.5, so now the direct stress at the corners would be (.sigma. being the direct stress): EQU .sigma.=2.25E.alpha..DELTA.T
This stress is now larger than the simple model used for the circular joint. In fact a better representation of the rectangular joint would produce an even higher stress due to twisting of the corners caused by the non-symmetrical shape of the cross section.